Some of currently used internal combustion engines for vehicles include an EGR device that is capable of recirculating part of exhaust gas to the intake side for the purpose of improving emissions performance. The EGR device includes an EGR valve, which is installed in an EGR path that connects an intake path of an internal combustion engine to an exhaust path, and controls the degree of opening of the EGR valve to adjust the amount of exhaust gas recirculated from the exhaust side to the intake side. A certain abnormality may occur in the EGR device so that it is unable to close or open the EGR valve. An abnormality in the EGR device affects the emissions performance. Therefore, when such an abnormality occurs, it should be immediately addressed or reported to a driver. In such an instance, an abnormality detection device for an EGR device that is described, for instance, in JP-A-2002-227727 is required. The device described in JP-A-2002-227727 (hereinafter referred to as the conventional device) estimates an intake pressure generated during a normal operation of the EGR device based on a throttle opening, an EGR valve opening, and an internal combustion engine speed, and then calculates the deviation between the estimated intake pressure and an actual intake pressure. When the deviation is not smaller than a predetermined value, the conventional device concludes that the EGR device is abnormal.
Meanwhile, the EGR device is also mounted on an internal combustion engine with a supercharger. The internal combustion engine with a supercharger is characterized in that its intake pressure is higher than back pressure in a certain operating region. A particularly serious abnormality that may occur in the EGR device mounted on such an internal combustion engine with a supercharger is the inability of the EGR valve to close due to trapped foreign matter or a fault. This causes the EGR valve to remain open. If the EGR valve is unable to close, fresh air may flow backward from the intake path to the exhaust path in a certain operating region of the internal combustion engine, and then flow into an EGR catalyst and a start catalyst, thereby causing the catalysts to overheat. In view of the above circumstances, it is demanded that the abnormality detection device for the EGR device mounted on the internal combustion engine with a supercharger accurately detect the inability of the EGR valve to close.
However, it is difficult for the aforementioned conventional device to fulfill such a demand. The reason is that the conventional device detects an abnormality in accordance with the deviation between the estimated intake pressure and the actual intake pressure. More specifically, even if the EGR device is abnormal, there is no significant deviation between the estimated intake pressure and the actual intake pressure in an operating region where the intake pressure is higher than the back pressure. In a certain operating region, therefore, the conventional device may fail to detect an abnormality in the EGR device. This may result in delayed abnormality detection.